• JSP源码、改写Servlet为JSP、查看转译成为Servlet的文件、JSP字符编码设置

    概述

    　　在Servlet中编写HTML太麻烦了，应该使用JSP。JSP中可以直接编写HTML，使用指示、声明、脚本（scriptlet）等元素来堆砌各种功能，但JSP最后还是会被容器转译为Servlet源代码、自动编译为.class文件、载入.class文件，然后生成Servlet对象。
    　　在第一次请求JSP时，容器会进行转译、编译与加载的操作，所以第一次请求JSP页面会慢很多。

    源码分析

    　　若使用Tomcat 7或Glassfish v3作为Web容器，最后由容器转译后的Servlet类是继承自HttpJspBase类，而HttpJspBase又继承了HttpServlet。
    　　下面是HttpJspBase类的源码，这个类的源码可以在Tomcat的lib目录下的 jasper.jarorgapachejasper untime 这个文件夹找到。

    package org.apache.jasper.runtime;

import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.jsp.HttpJspPage;
import org.apache.jasper.compiler.Localizer;

public abstract class HttpJspBase extends HttpServlet implements HttpJspPage {
    private static final long serialVersionUID = 1L;
    protected HttpJspBase() {
    }
    public final void init(ServletConfig config) throws ServletException {
        super.init(config);
        this.jspInit();
        this._jspInit();
    }
    public String getServletInfo() {
        return Localizer.getMessage("jsp.engine.info");
    }
    public final void destroy() {
        this.jspDestroy();
        this._jspDestroy();
    }
    public final void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this._jspService(request, response);
    }
    public void jspInit() {}
    public void _jspInit() {}
    public void jspDestroy() {}
    protected void _jspDestroy() {}
    public abstract void _jspService(HttpServletRequest var1, HttpServletResponse var2) throws ServletException, IOException;
}
    View Code 

    　　观察HttpJspBase的源代码，可以看到，Servlet的init()方法中调用了jspInit()与_jspInit()；Servlet的destroy()方法中调用了jspDestroy()与_jspDestroy()方法；service()方法中调用了_jspService()方法。
    　　_jspInit()、_jspDestroy()与_jspService()方法名称上有个下划线，表示这些方法是由容器转译时维护，不应该重新定义这些方法。如果想要做些JSP初始化或收尾动作，则应定义jspInit()或jspDestroy()方法。

    　　观察HttpJspBase的源代码，还可以看到，它继承了HttpServlet。但没有重写其doGet、doPost方法；只重写了HttpServlet参数为HttpServletRequest和HttpServletResponse的service方法，然后调用自己定义的抽象方法_jspService()。再观察jsp转译后的servlet，它的_jspService()方法内部并没有判断get、post方法，所以jsp既可以使用post也可以使用get来发出请求。

    Servlet 改写为 JSP

    将要被改写的Servlet源码：

    package com.test;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

@WebServlet("/books")
public class ListBookmark extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head>");
        out.println("<meta charset="UTF-8">");
        out.println("<title>觀看線上書籤</title>");
        out.println("</head>");
        out.println("<body>");
        out.println("<table style="text-align: left;  100%;" border="0" >");
        out.println(" <tbody>");
        out.println("<tr>");
        out.println("<td style="background-color: rgb(51, 255, 255);">书名</td>");
        out.println("<td style="background-color: rgb(51, 255, 255);">分类</td>");
        out.println("</tr>");

        Map<String, String> bookMap = new HashMap<String, String>();
        bookMap.put("三国演义", "小说");
        bookMap.put("资治通鉴", "历史");
        bookMap.put("窦娥冤", "戏剧");
        Set<Map.Entry<String, String>> entries = bookMap.entrySet();
        for (Map.Entry<String, String> entry : entries) {
            out.println("<tr>");
            out.println("<td>" + entry.getKey() + "</td>");
            out.println("<td>" + entry.getValue() + "</td>");
            out.println("</tr>");
        }

        out.println("</tbody>");
        out.println("</table>");
        out.println("</body>");
        out.println("</html>");
        out.close();
    }
}
    View Code

    手动改写好的JSP源码：

    <%@page contentType="text/html; charset=GBK" pageEncoding="UTF-8" %>
<%@page import="javax.servlet.*, java.io.PrintWriter, java.util.HashMap, java.util.Map, java.util.Set" %>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>在线书籍</title>
</head>
<body>
<table style="text-align:left; 100%;" border="0">
    <tbody>
    <tr>
        <td style="background-color: rgb(51, 255, 255);">书名</td>
        <td style="background-color: rgb(51, 255, 255);">分类</td>
    </tr>
    <%
        Map<String, String> bookMap = new HashMap<String, String>();
        bookMap.put("三国演义", "小说");
        bookMap.put("资治通鉴", "历史");
        bookMap.put("窦娥冤", "戏剧");
        Set<Map.Entry<String, String>> entries = bookMap.entrySet();
        for (Map.Entry<String, String> entry : entries) {
    %>
    <tr>
        <td><%= entry.getKey() %></td>
        <td><%= entry.getValue()%></td>
    </tr>
    <%
        }
    %>
    </tbody>
</table>
</body>
</html>
    View Code

    运行结果完全一致。

    index_jsp.java源文件：

    在IDEA中，JSP转译为Servlet后的源代码和进一步编译后的字节码文件可以在 target/tomcat/work/Tomcat/localhost/_/org/apache/jsp/ 这个文件夹下找到。

    /*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.47
 * Generated at: 2018-12-05 16:02:28 UTC
 * Note: The last modified time of this file was set to
 *       the last modified time of the source file after
 *       generation to assist with modification tracking.
 */
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
import javax.servlet.*;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  private static final javax.servlet.jsp.JspFactory _jspxFactory =
          javax.servlet.jsp.JspFactory.getDefaultFactory();

  private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;

  private javax.el.ExpressionFactory _el_expressionfactory;
  private org.apache.tomcat.InstanceManager _jsp_instancemanager;

  public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
    return _jspx_dependants;
  }

  public void _jspInit() {
    _el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
    _jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
  }

  public void _jspDestroy() {
  }

  public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
        throws java.io.IOException, javax.servlet.ServletException {

    final javax.servlet.jsp.PageContext pageContext;
    javax.servlet.http.HttpSession session = null;
    final javax.servlet.ServletContext application;
    final javax.servlet.ServletConfig config;
    javax.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    javax.servlet.jsp.JspWriter _jspx_out = null;
    javax.servlet.jsp.PageContext _jspx_page_context = null;


    try {
      response.setContentType("text/html; charset=GBK");
      pageContext = _jspxFactory.getPageContext(this, request, response,
                  null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

      out.write("
");
      out.write("
");
      out.write("<!DOCTYPE html>
");
      out.write("<html>
");
      out.write("<head>
");
      out.write("    <meta charset="UTF-8">
");
      out.write("    <title>在线书籍</title>
");
      out.write("</head>
");
      out.write("<body>
");
      out.write("<table style="text-align:left; 100%;" border="0">
");
      out.write("    <tbody>
");
      out.write("    <tr>
");
      out.write("        <td style="background-color: rgb(51, 255, 255);">书名</td>
");
      out.write("        <td style="background-color: rgb(51, 255, 255);">分类</td>
");
      out.write("    </tr>
");
      out.write("    ");

        Map<String, String> bookMap = new HashMap<String, String>();
        bookMap.put("三国演义", "小说");
        bookMap.put("资治通鉴", "历史");
        bookMap.put("窦娥冤", "戏剧");
        Set<Map.Entry<String, String>> entries = bookMap.entrySet();
        for (Map.Entry<String, String> entry : entries) {
    
      out.write("
");
      out.write("    <tr>
");
      out.write("        <td>");
      out.print( entry.getKey() );
      out.write("</td>
");
      out.write("        <td>");
      out.print( entry.getValue());
      out.write("</td>
");
      out.write("    </tr>
");
      out.write("    ");

        }
    
      out.write("
");
      out.write("    </tbody>
");
      out.write("</table>
");
      out.write("</body>
");
      out.write("</html>
");
    } catch (java.lang.Throwable t) {
      if (!(t instanceof javax.servlet.jsp.SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try { out.clearBuffer(); } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
        else throw new ServletException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}
    View Code

    JSP响应内容、字符编码设置 

    jsp源文件：　　　　<%@page contentType="text/html; charset=GBK" pageEncoding="UTF-8" %>
    index_jsp.java：　　response.setContentType("text/html; charset=GBK");

    jsp源文件：　　　　<%@page contentType="text/html" pageEncoding="UTF-8" %>
    index_jsp.java：　　response.setContentType("text/html;charset=UTF-8")

    jsp源文件：　　　　<%@page contentType="text/html;" pageEncoding="UTF-8" %>
    index_jsp.java：　　response.setContentType("text/html;;charset=UTF-8")

    前两种方法都可以，第三种方法的index_jsp.java里多个一个分号 response.setContentType("text/html;;charset=UTF-8")

    pageEncoding属性用于告诉容器使用UTF-8读取.jsp转译为.java，然后编译时也使用UTF-8，并且如果在contentType属性里没有指定字符编码的情况下设置内容类型附加的charset也为UTF-8。
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  • 原文地址：https://www.cnblogs.com/Mike_Chang/p/10073990.html
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